The present invention relates to a displacement control mechanism for a variable displacement type compressor which adjusts the pressure in a pressure control chamber by supplying refrigerant gas in a discharge-pressure region into the pressure control chamber and releasing the refrigerant gas in the pressure control chamber to a suction-pressure region, thereby controlling the displacement of the compressor.
In a variable displacement type compressor provided with a pressure control chamber having therein a swash plate whose inclination angle is variable, the inclination angle of the swash plate decreases with an increase of the pressure in the pressure control chamber. On the other hand, the inclination angle of the swash plate increases with a decrease of the pressure in the pressure control chamber. When the inclination angle of the swash plate decreases, the stroke of a piston decreases thereby to decrease the displacement of the compressor. When the inclination angle of the swash plate increases, the stroke of the piston increases thereby to increase the displacement of the compressor.
Since the refrigerant gas which is supplied to the pressure control chamber has been already compressed, the operating efficiency of the variable displacement type compressor deteriorates as the amount of refrigerant gas released from the pressure control chamber to a suction-pressure region of the compressor increases. Therefore, the cross-sectional area of a release passage through which the refrigerant gas is released from the pressure control chamber to the suction-pressure region should be small as much as possible in view of the operating efficiency.
If the compressor is left in a stopped state for a long time, the refrigerant gas is changed into a liquid state and the liquefied refrigerant is accumulated in the pressure control chamber. When the compressor is started in such a state, the liquefied refrigerant is not released rapidly to the suction-pressure region if the release passage has a fixed throttle with a small cross-sectional area. As a result, the liquefied refrigerant is vaporized in the pressure control chamber and the pressure in the pressure control chamber is increased excessively. Therefore, it takes a long time before the displacement of the compressor is increased to a desired level after the compressor is started
A variable displacement type compressor with a displacement control mechanism is disclosed in Japanese Patent Application Publication NO. 2002-21721 to solve the above problem. The displacement control mechanism in this Publication has a first control valve which adjusts the cross-sectional area of a refrigerant gas supply passage through which refrigerant gas is supplied from a discharge-pressure region of the compressor to the pressure control chamber and a second control valve which adjusts a cross-sectional area of a refrigerant gas release passage through which refrigerant gas is released from the pressure control chamber to a suction-pressure region of the compressor The first control valve is an electromagnetic control valve which is operable to adjust the opening degree of the valve by changing the electromagnetic force. When the first control valve is in deenergized state, the opening degree of the valve is maximum and the inclination angle of a swash plate is minimum. This state corresponds to the minimum displacement operation of the compressor in which the displacement thereof is fixed at minimum. When the first control valve is in energized state, the opening degree of the valve becomes smaller than the maximum and then the inclination angle of the swash plate becomes larger than the minimum. This state corresponds to an intermediate displacement operation in which the displacement is not fixed to the minimum.
The second control valve has a spool (a valve body for adjusting the cross-sectional area of the release passage) defining a cylindrical space and a back pressure chamber in the spool chamber in which the spool is accommodated. The back pressure chamber communicates with a pressure region downstream of the first control valve and the cylindrical space communicates with the pressure control chamber through a release passage (bleed passage). The spool is urged toward the back pressure chamber by a spring. A bleed hole is formed in the spool so as to secure a minimum cross-sectional area of the release passage. When the variable displacement type compressor is started, the first control valve is closed and the spool of the second control valve is moved in direction which increases the cross-sectional area of the release passage. Thus, the liquefied refrigerant in the pressure control chamber is rapidly released to the suction-pressure region, thereby reducing the time before the displacement is increased to a desired level after the variable displacement type compressor is started
When the first control valve is in energized state and opened, the second control valve is closed (or its spool is seated against a valve seat) and the refrigerant gas is released from the pressure control chamber to the suction-pressure region only through the bleed hole. In this state, the compressor is operating under a displacement more than the minimum (i.e. intermediate displacement).
When the cross-sectional area of the bleed hole is adjusted to be small, the pressure in the cylindrical space when the second control valve is in the closed state becomes substantially the same as that in the pressure control chamber. Since the first control valve has a throttling function, the pressure in the back pressure chamber becomes a pressure corresponding to the pressure in the pressure control chamber that is slightly higher than that in the cylindrical space.
Since the refrigerant gas released from the pressure control chamber to the suction chamber needs to be stopped during compressor operation under the minimum displacement, the second control valve should be in the closed state (or the spool be seated against the valve seat). Furthermore, the pressure in the back pressure chamber is slightly higher than that in the cylindrical space. Accordingly, the spring force of the spool spring needs to be small so that the spool is seated against the valve seat by the differential pressure between the back pressure chamber and the cylindrical space during the compressor operation under the minimum displacement.
When the first control valve is changed from the opened state to the closed state, the spool is moved away the valve seat. If the spring force of the spool spring is too small, however, the spool movement may be hampered by any foreign matters present between the peripheral surface of the spool and its accommodation chamber This prevents the liquefied refrigerant in the pressure control chamber from being rapidly released when the compressor is started.
If the cross-sectional area of the bleed hole is made too large, an excessive amount of refrigerant gas is released from the pressure control chamber to the suction chamber, with the result that the operating efficiency is deteriorated. Therefore, the present invention is directed to providing a variable displacement type compressor with a displacement control mechanism according to which the time taken before the displacement of the compressor is increased to the desired level after a start-up of the compressor is reduced and also the operating efficiency of the compressor is improved.